Defibrillators get less shocking

August 5, 2004By Mark PeplowThis article courtesy of Nature News.

Implant targets scar tissue to kick-start heart more gently.

For the millions of people who carry a defibrillator inside their chest, cheating death is a shocking experience. But a technique that manipulates the rotating electrical waves that cause fibrillation could dramatically reduce the electrical jolt needed to get a heart pumping properly again.

Igor Efimov, an electrophysiologist at Case Western Reserve University in Cleveland, Ohio, says his finding could lead to defibrillators that cause less damage to the heart when they discharge. This would lift users' anxiety of knowing that a painful, albeit life-saving, shock could be triggered at any moment. One patient with a defibrillator describes the jolt delivered by today's devices as "like being kicked in the chest - it's certainly not very pleasant".

During fibrillation, the heart cells stop contracting in synchrony and fire randomly, reducing the heart to a quivering lump of meat. Without intervention to get the cells marching in step once more, death is imminent.

Defibrillation is currently a crude technique, which delivers an electric current of between 3 and 10 joules of energy to disrupt the rogue waves and shock the heart back into its normal activity.

Roughly 175,000 people around the world have a defibrillator fitted inside their chest every year: the first was fitted in 1980. The devices are usually fitted just below the breast bone, with electrical leads going into the heart.

Cardiac tornado

“The psychological effect of getting shocks is one of the most overlooked problems of these devices”

Patrick Tchou Cleveland Clinic Heart Centre, Ohio

More than 80% of patients fitted with defibrillators have already had a heart attack. Heart attacks increase the risk of fibrillation because they leave scars of dead tissue inside the heart, which have a higher electrical resistance than the surrounding tissue. This allows rotating electrical waves to develop around these areas, which disrupt the heart's normal electrical activity. "They are like a tornado," says Efimov.

Efimov has modelled these waves to come up with a gentler way to restart the heart. By timing the shock so that it arrives just as the wave is passing around the scar tissue, he says it is possible to dislodge and quench the electrical tornado using only half a joule of energy - so little that a patient would barely notice it.

The approach works because scar tissue is more susceptible to electric shocks, so a small pulse affects only the scar, not the surrounding cells. If the electrical pulse is applied at just the right time, it dislodges the wave from the scar tissue. Once freed into the surrounding healthy tissue, the wave can no longer sustain itself and dies away.

Efimov has tested the idea in rabbit hearts, and managed to defibrillate them with less than 20% of the energy that is normally required. Calculations suggest that this could be reduced further as the technique is refined, he reports in the latest issue of Physical Review Letters.

Size matters

Using less energy will avoid problems caused by conventional defibrillators, which can make tiny holes in the membranes of heart cells, says Efimov. Most of these holes repair themselves, but it can be dangerous for patients who already have the most damaged hearts.

But the most important benefit for patients could be a psychological one. "The psychological effect of getting shocks is one of the most overlooked problems of these devices," says Patrick Tchou, a cardiologist at the Cleveland Clinic Heart Centre, Ohio. "It packs quite a kick, so any technique that can reduce the energy of shocks will make it much more tolerable."

"It's early days yet, but we've all been hoping for a while that we could use less energy in defibrillation," says Stuart Cobbe, a British Heart Foundation cardiologist based at the University of Glasgow, UK.

If less energy were required, manufacturers could also make the devices much smaller, Cobbe adds. Defibrillators are currently around the size of a packet of cigarettes, not insignificant for a device that is implanted into the chest. "If this works out, it could make a defibrillator the same size as a pacemaker - that's about a fifth the size," he says.

Efimov is now collaborating with Medtronic, one of the world's biggest manufacturers of defibrillators, based in Minneapolis, Minnesota, to turn his laboratory studies into a medical reality. The new defibrillators could be developed within three to five years, says Walt Olson, senior director of implantable cardiac defibrillator research at Medtronic.